16 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
17 oriented protocol designed to solve issues present in UDP and TCP, particularly
18 for real-time and multimedia (streaming) traffic.
19 It divides into a base protocol (RFC 4340) and plugable congestion control
20 modules called CCIDs. Like plugable TCP congestion control, at least one CCID
21 needs to be enabled in order for the protocol to function properly. In the Linux
22 implementation, this is the TCP-like CCID2 (RFC 4341). Additional CCIDs, such as
23 the TCP-friendly CCID3 (RFC 4342), are optional.
24 For a brief introduction to CCIDs and suggestions for choosing a CCID to match
25 given applications, see section 10 of RFC 4340.
27 It has a base protocol and pluggable congestion control IDs (CCIDs).
29 DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
30 is at http://www.ietf.org/html.charters/dccp-charter.html
35 The Linux DCCP implementation does not currently support all the features that are
36 specified in RFCs 4340...42.
38 The known bugs are at:
39 http://linux-net.osdl.org/index.php/TODO#DCCP
41 For more up-to-date versions of the DCCP implementation, please consider using
42 the experimental DCCP test tree; instructions for checking this out are on:
43 http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
49 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
50 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,
51 the socket will fall back to 0 (which means that no meaningful service code
52 is present). On active sockets this is set before connect(); specifying more
53 than one code has no effect (all subsequent service codes are ignored). The
54 case is different for passive sockets, where multiple service codes (up to 32)
55 can be set before calling bind().
57 DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet
58 size (application payload size) in bytes, see RFC 4340, section 14.
60 DCCP_SOCKOPT_AVAILABLE_CCIDS is also read-only and returns the list of CCIDs
61 supported by the endpoint (see include/linux/dccp.h for symbolic constants).
62 The caller needs to provide a sufficiently large (> 2) array of type uint8_t.
64 DCCP_SOCKOPT_SERVER_TIMEWAIT enables the server (listening socket) to hold
65 timewait state when closing the connection (RFC 4340, 8.3). The usual case is
66 that the closing server sends a CloseReq, whereupon the client holds timewait
67 state. When this boolean socket option is on, the server sends a Close instead
68 and will enter TIMEWAIT. This option must be set after accept() returns.
70 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the
71 partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums
72 always cover the entire packet and that only fully covered application data is
73 accepted by the receiver. Hence, when using this feature on the sender, it must
74 be enabled at the receiver, too with suitable choice of CsCov.
76 DCCP_SOCKOPT_SEND_CSCOV sets the sender checksum coverage. Values in the
77 range 0..15 are acceptable. The default setting is 0 (full coverage),
78 values between 1..15 indicate partial coverage.
79 DCCP_SOCKOPT_RECV_CSCOV is for the receiver and has a different meaning: it
80 sets a threshold, where again values 0..15 are acceptable. The default
81 of 0 means that all packets with a partial coverage will be discarded.
82 Values in the range 1..15 indicate that packets with minimally such a
83 coverage value are also acceptable. The higher the number, the more
84 restrictive this setting (see [RFC 4340, sec. 9.2.1]). Partial coverage
85 settings are inherited to the child socket after accept().
87 The following two options apply to CCID 3 exclusively and are getsockopt()-only.
88 In either case, a TFRC info struct (defined in <linux/tfrc.h>) is returned.
89 DCCP_SOCKOPT_CCID_RX_INFO
90 Returns a `struct tfrc_rx_info' in optval; the buffer for optval and
91 optlen must be set to at least sizeof(struct tfrc_rx_info).
92 DCCP_SOCKOPT_CCID_TX_INFO
93 Returns a `struct tfrc_tx_info' in optval; the buffer for optval and
94 optlen must be set to at least sizeof(struct tfrc_tx_info).
96 On unidirectional connections it is useful to close the unused half-connection
97 via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
101 Several DCCP default parameters can be managed by the following sysctls
102 (sysctl net.dccp.default or /proc/sys/net/dccp/default):
105 The number of active connection initiation retries (the number of
106 Requests minus one) before timing out. In addition, it also governs
107 the behaviour of the other, passive side: this variable also sets
108 the number of times DCCP repeats sending a Response when the initial
109 handshake does not progress from RESPOND to OPEN (i.e. when no Ack
110 is received after the initial Request). This value should be greater
111 than 0, suggested is less than 10. Analogue of tcp_syn_retries.
114 How often a DCCP Response is retransmitted until the listening DCCP
115 side considers its connecting peer dead. Analogue of tcp_retries1.
118 The number of times a general DCCP packet is retransmitted. This has
119 importance for retransmitted acknowledgments and feature negotiation,
120 data packets are never retransmitted. Analogue of tcp_retries2.
123 Whether or not to send NDP count options (sec. 7.7.2).
126 Whether or not to send Ack Vector options (sec. 11.5).
129 The default Ack Ratio (sec. 11.3) to use.
132 Default CCID for the sender-receiver half-connection.
135 Default CCID for the receiver-sender half-connection.
138 The initial sequence window (sec. 7.5.2).
141 The size of the transmit buffer in packets. A value of 0 corresponds
142 to an unbounded transmit buffer.
144 sync_ratelimit = 125 ms
145 The timeout between subsequent DCCP-Sync packets sent in response to
146 sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
147 of this parameter is milliseconds; a value of 0 disables rate-limiting.
152 Works as in udp(7): returns in the `int' argument pointer the size of
153 the next pending datagram in bytes, or 0 when no datagram is pending.
158 DCCP does not travel through NAT successfully at present on many boxes. This is
159 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
160 support for DCCP has been added.